Multiplier phototube circuit



Sept. 2, 1958 J. R. PARSONS MULTIPLIER PHOTOTUBE CIRCUIT ,FiledMay 3, 1955 umd Tx Si INVENTOR. cflz ZQPJOHJ BY ATTORNEY:

Unite 2,850,644 MULTnaLIEn PHororUBE CIRCUIT Application May 3, 1955, Serial hlm-505,662

4 Claims. (Cl. Z50-207) This invention relates generally to multiplier phototube circuits, and more particularly to a light detecting circuit comprising means to stabilize it against variations in the intensity of the exciting light source, and against variations in the operating voltage. While neither specically nor exclusively limited thereto, the multiplier phototube circuit of the present invention is particularly useful in a portable instrument of the type employed to detect changes in color of a sensitive chemical indicator when alected by a gas or vapor.

It has been proposed to stabilize multiplier phototube circuits against unwanted changes in the intensity of the exciting light source, and against variations in the voltage of the voltage source by varying the dynode voltages applied to the dynodes of the multiplier phototubes to vary their gain accordingly. In such circuits, at least two multiplier phototubes have to be matched carefully because their operating characteristics have to be similar to each other over substantially the entire range of the phototubes operating characteristic curves. When the gains of the multiplier phototubes are varied to compensate for the aforementioned changes, the sensitivities of the multiplier phototubes vary. Thus, multiplier phototube circuits that are compensated by Varying the gains of the multiplier phototubes have an output voltage that will vary with respect to a reference voltage even in the absence of an actual light signal to be measured. Hence, such a varying output voltage cannot be used to operate a subsequent indicating circuit at its optimum transconductance point. Y

Accordingly, it is a principal object of the present invention to provide an improved multiplier phototube circuit that will be stabilized against variations in operating voltages, and in unwanted variations in the intensity of the light source, and yet provide an output voltage that is xed with respect to a reference voltage, in the absence of an actual signal.

Another object of the present invention is to provide an improved multiplier phototube circuit, employing multiplier phototubes that are always operated at a constant gain, and having means to stabilize it against the aforementioned variations.

A further object of the present invention is to provide an improvedmultiplier phototube circuit employing multiplier phototubes that need not be matched, and employing means to operate an output circuit in a highly ecient manner.

It is still a further object of the present invention to provide an improved multiplier phototube circuit employing phototubes that always operate with a dxed gain and have an output voltage that is held substantially constant with respect to a reference voltage in the absence of actual signals to be detected.

A still further object of the present invention is to provide an improved multiplier phototube circuit that is relatively simple in construction and operaiton, economical to manufacture and yet highly ecient in use.

atent "i Patented Sept. 2, 1958 These and, perhaps, further objects of the present invention are attained in an improved multiplier phototube circuit having at least two multiplier phototubes. Each ofthe multiplier phototubes has a photocathode, one or more intermediate dynodes and an anode. The photocathodes and the dynodes of the multiplier phototubes are connected in parallel to each other, respectively, and the dynodes are provided with operating voltages from a voltage divider connected across a source of regulated relatively high unidirectional voltage. Thus, the gain of each of the multiplier phototubes is maintained constant within the range of normal operating conditions. Means are provided to maintain each of the anodes of each of the multiplier phototubes at a fixed voltage with respect to a reference voltage, such as ground, in the absence of an actual signal to be detected. To this end, a rst circuit comprising a first variable impedance tube and a cathode resistor, and a second circuit cornprising a second variable impedance tube and an anode resistor are connected in parallel between ground and a source of relatively low unidirectional voltage. The anode of the second variable impedance tube is connected to the grid ofthe rst variable impedance tube, and the cathode resistor is connected to the anode of each multiplier phototube through a separate load resistor. The anode of one of the multiplier'phototubes, hereinafter referred to as a reference multiplier phototube, is connected to the grid of the second variable impedance tube. The anode of the other of the multiplier phototubes, hereinafter called the sampling multiplier phototube, comprises the output of the sampling multiplier phototube.v Similar variations in the intensity of the exciting light source sensed by both the reference and the sampling multiplier phototubes, will produce no change in the anode voltage of the sampling multiplier phototube with respect-to ground. Also, changes in the voltage source, within a range of normal operation, will not produce any change in the anode voltage of the sampling multiplier phototube.

A change in the output voltage, that is, the anode voltage, of the sampling multiplier phototube will result only when the sampling multiplier phototube receives an actual light signal. An actual light signal as used herein refers to a change in the light intensity caused by an event that is to be detected, such as the change in color of a chemically treated indicator material in the presence of a gas.

The novel features of the invention, as well as the invention itself, both as to its organization and method of operation, will be understood in detail from the following description when considered in connection with the accompanying drawing, the single igure of which is a schematic illustration of a multiplier phototube circuit in accordance with the present invention.

Referring now to the drawing, there is shown a multiplier phototube circuit comprising a reference multiplier phototube 10 and a sampling multiplier phototube 12. The photocathodes i4 and 16 of the multiplier phototubes 10 and 12 are connected to each other and to a negative terminal 18 for applying a relatively high unidirectional negative voltage, say -LOGO volts, thereto. Each of the multiplier phototubes itl and 12 are provided with one or more dynodes. The dynodes Ztl-22 of the reference multiplier phototube lil are connected in parallel with the dynodes 23-25 of the sampling multiplier phototube 12, respectively, and to successive points along a voltage divider 26 for the purpose of-providing iixed operating voltages thereto. The voltage divider 26 is connected between the terminal 1S and a point of reference voltage, such as ground. While only three dynodes are shown for each of the multiplier phototubes l@ and 12, it is understood that the number of dynodes employed will be determined by the sensitivity desired.

theY multiplier phototubes constant.

*Means are provided to maintain the gain of each o To this end, regulated voltage means arejprovided Vbetween the terminal 18 and ground to maintain the voltage across the voltage v YV-di-vider'26 Vsubstantially constant. VOperating voltages are Y provided byfay blocking oscillator circuitcomprising, for 'fexamplq af-triodetube 30. A source ofu'nidirectional "",voltage,'su`chfas `a 2,4 volt battery 32,"has its positive ter- 'minal connected to the anode of the-tube '30 through '.'"a-section'34 of an autotransformer 36.5 The fanode of V"a tube'30 is connected to the anode of a rectierl diode `3S through a'section 40 of the vautotransformer 36 and d'a'capacitoru42f The cathodeof the diode38 comprises j'ta iil'mentlconnected to groundY and to asource of'suita'A-'ableV operatingV voltage (not shown), in Aa lmanner Vwell '-'fknown in "the art. t The ilament and heater connections, of theftubesused herein, are not shown Vbecauselthey arewellknownrin the art, and'becausre it is 4desired to maintain-theV diagram as simple'and readable'as possible. The negative terminal of the voltage source 32 is vconnected to the cathode of the tube 30, and to ground.

The grid of the tube is connected to ground through Va section 44 of an autotransformer 46 wound on the same core as the autotransformer 376. The section 44 of theV autotransformer 46 is also connected to ground Each end of the lter l A voltage regulating'tube62, of the gas type, is connectedbetween the terminal 18 and ground for the purpose Vof maintainf VingV the unidirectional voltage therebetween substantially i `Yconstant. n

It will now be understood that the tube 30 isrconfv nected in a blocking oscillator circuit for converting Ythe Vi'elatively low batteryvoltage 32 into an alternating'voltage, and rectifying thisV alternating voltage by the recti- "j ersf and 54 to provide a relatively high unidirectional 'l voltage VYbetween ythe output terminal 18 and ground.-y

A-voltage of about -l,000 volts, with respect to ground, is obtained at the negative terminal 18 inthe following manner:` Current will flow throughl the ytube 30 until the saturation point is reached. Upon saturation, the grid ofthe tube 50 will be Vdriven sufiicientlynegative so thatV conduction through the tube 30 .will be cut off. VThis causesY a collapse of the magnetic fields about. theauto- Vtransformers 36'and 46, and induces a stepped up voltage ithereacross.' Theegrid of the tube 30 will now approach ground potential and Acause current to flow through the tube 30 again, Yrepeating'the cycle. The autotransformer 46, the capacitor 42, and the rectiers 38 and 54 function inthe nature of a voltage multiplier circuit to produce a relatively highV voltage across the capacitor 42. This Vvoltage is ltered'by the resistor-capacitor network 56, 58 and 60,V in the Yusual manner,.regulated byjthervoltage f regulating'tube 62, andY applied between the Vterminal 18 fand ground.

The alternating voltage inducedrin the autotransformer Y46 is rectied by the rectier 50, and the rectitled'voltage is impressed across the capacitor 52'. Y Thus, a unidirec- I f tional voltage that is positive with respectY torground may be Yobtained at a positive terminal 64, connected between the rectifier VStirandV the capacitor 52. The unidirectional voltage appearing between the positive terminal 64 and ,Y ground isrrela'tively low, say l0() volts, `compared to the unidirectional voltage'betwee'n; ground and thenegative terminl 18.Y

l "i The Vblocking oscillator, comprising the tube 311 and its Y associated vcircuitry, Vmay be V*looked upon asfa power supply for converting the unidirectional voltage source 32 'into aV relatively loaI `unidirectional voltage,V say V100 volts, between the positive Vterminals 64 and ground, and

`into a relatively high negative unidirectional voltage, say

1,000 volts, between "ground and the negative terminal 18.

the positive terminal V64, ,and the cathode,V thereof is connected'to ground through a c'athode'resistor 70. The anode of theftriode tube 68 is-connected to the positive terminal 64 through an anode resistor 72, and the cathode thereof is connected to ground. The cathode of the Y tube 68V is of the ilament type; the source of iilament voltage beingk omittedifor purposes of clarification. The

anode of the tube 68 `is connectedV to .thel grid of the l V,tube 66.` The cathode ofthe tuber'iis'connected to i' the anodes of the .multiplierv phototubes 10 and 12through the separate'a'node resistorsk 74 and 76,Y respectively. VThe grid; of the tube 68 is'connected to `the anode of the reference multiplier phototubeV 10. f

Y The multiplierphototubes 10 and 12- are physically disposed to` receive light from adjacent chambers'80 and 82, respectively. Appropriate light directing shades(not Vshown) may be used."V The dashed lines 84 and' 86'reprei `The chambers 80 and 82 are-'formedy with A'a light transparent llter 88 sothatreiiected lightfrom the'chanbers 80 a`ndr82'may pa'ss `to the multiplier phototubes 1K0 andV 12.V Af'strip'of chemically treated indicator material90 is disposed atgthe bottom Vof the chambers 80 Aand 82 in'a mannerV Whereby'reected light therefromjvwillfpass Y through the filter 88 andA onto the'phototubes 410 and 12.

' The indicator k,material 90, for example, maybefsiinilar to litmus `-`paper whose color is dependent uponthelalkalintyor'ac'idity of vapors yto which it is subjected; It

will be understoodv thatA the chambers 80'and 82 are illuminated equally by theh same llight'source (not shown) s'o'fthatthe light reflected from the indicator `material 90-to-t`he'phototubes 10 and k1,2"will'be the same-when Y the indicatorY material 90 is-unaie'cted-by anyjvapor' or' gas. Y

The chamber 82 is formed with Van opening92the`rein forY the Vpurpose of allowing a gas orV vapor Yto'enter.

whereby to aiect the indicator material 90 Vwithin the chamber S2 only.V It will also be understood that the chamber 80 is sealed in a manner whereby noV gas to AAVdetected, will 'affect each ,of the phototubes 10 Yand 12in `-`Va similar manner; An actualsignal, due to the presenceV of aV gas or vapor in the'chamber S2 only, `will cause the Vportion of the indicatormaterial 'within the chamber 82 yto change color in a manner whereby the signal re'- YV,ceived by the vsampling'phototube 12 Vwill be different to the grid of a triode tube 94. VVT he tube 94 and a'triodeV Y tube 96 are connected infa bridge circuit` 98 that fuuctions as an indicating means to'findicate the presencer of a gas or vapor to be detected. The triode tubes 94-and 96 Vmay be enclosed in .the'same envelope, as a duotriode.V VThe anodes of'the` tubes 94 andv 96 areY connectedto aT source Vof suitable operatingzvoltagqsuch as the voltage p available at'the positive outputf-terminal64qf The cathi odes ofthe tubes V94 and 96 are connected tot ground lsentthe direction of light from the chambers 80 and 82 tothe multiplier vphototubes 10 and 12,1 respectively.

between the cathodes of the tubes 94 and 96. The grid of the tube 96 is connected to a suitable point on the cathode resistor 102 in order to ix its bias and to cause the tube 96 to conduct uniformly.

In order to derive an indication on the voltmeter 104, under conditions of maximum sensitivity, the tube 94 must be operated at its optimum transconductance point. It is an important feature of the present invention to maintain the grid of the tube 94 at a predetermined xed potential with respect to ground, in the absence of an actual signal, due to the presence of a gas or vapor, so that the tube 94 will operate at its optimum transconductance point.

The operation of the multiplier Yphototube circuit, in accordance with the present invention, will now be described: Let it be assumed that because of aging and/or temperature changes, for example, there is a similar change in the indicating material 90 within the chambers 80 and 82 so that a similar change in the intensity of light is sensed by the reference multiplier phototube and by the sampling multiplier phototube 12. It will be understood that the gain of each of the multiplier phototubes 10 and 12 is maintained substantially constant because the voltage across the voltage divider 26 is maintained constant by the voltage regulator tube 62. If, for example, it is assumed now that the reflected light from the indicator material 9i) is decreased by these changes, there will be a decrease in the conduction through the reference multiplier phototube 10, and through the sampling multiplier phototube 12, and the anode voltages thereof will tend to go more positive. This positive-going signal will cause the grid of the tube 68 to go positive, and to provide a negative-going voltage at the grid of the tube 66. Because of the cathode follower action of the tube 66, this negative-going voltage is applied to the anodes of the phototubes 10 and 12 through their anode resistors 74 and 76, respectively. Thus, the original tendency of the anodes of the phototubes 10 and 12 to go positive are offset by this latter negative-going voltage. It will also be understood that any tendency for the anodes of the phototubes 10 and 12 to go negative, because of similar light signals received, will result in a positive-going signal at the cathode of the tube 66 that will tend to compensate for these changes.

It will also be noted that any change in the operating voltage of the tubes 66 and 68 will be compensated in a manner whereby the anode voltages of the phototubes 10 and 12 will tend to remain substantially constant, in the absence of a signal to be detected. Let it be assumed, for example, that the anode voltage `on the tubes 66 and 68 decreases. The cathode voltage of the tube 66 will decrease and cause the grid of the tube 68 to decrease in voltage. This will result in a positive-going voltage at the anode of the tube 68. This positive-going voltage is now applied to the grid of the tube 66, and conduction therethrough will now increase. An increased conduction through the tube 66 will now provide a positive-going signal at the cathode thereof. Thus, a tendency for the operating voltage across the tubes 66 and 68 to vary results in a set of conditions whereby the voltage across the cathode resistor 70 is maintained substantially constant.

The grid bias for the tube 94, in the absence of a signal due to a gas or a vapor to be detected by the sarnpling multiplier phototube 12 will be maintained at a fixed voltage whereby the tube 94 of the bridge circuit 98 will operate at its optimum transconductance point. It will be understood that the bias required for such optimum operation is obtained from the anode voltage of the tube 12. Let it now be assumed that the vapor or a gas to be detected enters the opening 92 whereby the portion of the indicator material 90 within the chamber 82 changes in color. Let it be assumed, also, that as a result of this change in color, the intensity of light sensed by the sampling multiplier phototube. 12 is decreased. This will result in decreased conduction through the multiplier phototube 12, and the anode voltage of the sampling multiplier phototube 12 will tend to go positive. The positive-going voltage is applied to the grid of the tube 94, thereby increasing conduction therethrough and raising the voltage at the cathode thereof. Since the grid bias for the tube 96 is lixed and conduction therethrough is steady, the meter 104 will iudicate any difference in voltage between the cathodes of the tubes 94 and 96. By maintaining the grid of the tube 94 at its optimum transconductance point in the absence of an actual signal, the occurrence of an actual signal due to a gas or Vapor, will provide a maximum amplication within the tube 94, thereby increasing the overall sensitivity of the circuit.

What is claimed is:

1. A multiplier phototube circuit comprising a pair 0f multiplier phototubes each having an anode, a photocathode and one or more intermediate dynodes, parallel connections between said photocathodes and said dynodes of said pair of multiplier phototubes, a point of reference voltage, means including a voltage divider to apply a source of regulated unidirectional voltage between each of said photocathodes and said point of reference vbltage, means connecting each of said dynodes to selected points on said voltage divider to x the voltage at said dynodes with respect to said point of reference voltage, a pair of resistors connected to each other and between the anodes of said pair of multiplier phototubes, a third resistor connected between the common junction of said pair of resistors and said point of reference potential, and means including said pair of resistors and said third resistor to maintain said anodes at a substantially constant voltage with respect to said point of reference potential when light signals received by each of said multipleir phototubes are the same.

2. A multiplier phototube circuit comprising a pair of multiplier phototubes each having an anode, a photocathode and one or more intermediate dynodes, parallel connections between said photocathodes and said dynodes of said pair of multiplier phototubes, a point of reference voltage, means to apply a source of regulated unidirectional voltage between each of said photocathodes and said point of reference voltage, means connecting each of said dynodes to said regulated voltage applying means to fix the voltage of said dynodes with respect to said point of reference voltage, a pair of resistors connected to each other and between the anodes of said pair of multiplier phototubes, a rst and a second tube each having at least an anode, a grid and a cathode, a cathode resistor connected between said cathode of said first tube and said cathode of said second tube, an anode resistor connected between said anode of said rst tube and said anode of said second tube, means to apply an operating voltage between said anode of said first tube and said cathode of said second tube, said cathode of said second tube being connected to said point of reference voltage, means connecting said cathode of said irst tube to the common junction of said pair of resistors; means connecting the anode of said second tube to the grid of said iirst tube, and means connecting the grid of said second tube to an anode of one of said pair of multiplier phototubes.

3. A multiplier phototube circuit comprising first and second multiplier phototubes each having a photocathode, one or more intermediate dynodes and an anode, a point of reference voltage, a voltage divider connected between said photocathodes and said point of reference voltage, means connecting said dynodes to points on said voltage divider, means for applying a source of xed voltage across said voltage divider, a pair of resistors connected to each other and between -said anodes, a first tube having an anode, a grid and a cathode, a cathode resistor connected between said cathode of said rst tube and said point of reference voltage a'second tube having an anode, a grid and acathode, said cathode of said second tube being connected to said pointof reference rvoltage, a`

loadfresistor connected betweenthe Vanode of saidf'irst tube and the anode Vof said second tube, means to apply f a source of unidirectionalV voltage between said'anode ofsaid irst tube and said point of reference voltage, said vcathode of said first tube being ccmnectedy to the common junction of said pairf of resistors, said anode of said second tube being rconnected `to said grid kof`said iirst tube,'and means connectingsaid grid of said second tube toV said anode of 'said irst multiplier phototube.

4. A multiplier photo. circuit, adapted to` compensateor variations in the'intensity of an exciting light source,

comprising a reference multiplier phototube and a sam-V pling multiplier phototube, each of said phototubes having a photocathode, one or more dynodes andananode, a-point of reference voltage, means to apply a xed voltageV between said photocathodes and'saidV point of refer-V ence voltage, said last-mentioned means comprising means to apply fixed voltages ,to said dynodes with respect to said point of reference voltage, means tomaintain each of said anodes-,at a constant voltage with respect to said point of referencevoltage comprising Aa rstand secondl said rst Ytube being connected toV said anodeY offsaid' referenceV multiplierV phototubevand to Ysaid anode of said4v sampling multiplier phototubevthrough a separate-oner- 0f said pair of resistors, respectively, and said gridot` said second tubepbeingconnected to the anode of said` reference multiplier phototube.

References Cited in the iile` of this patent Y UNITED STATES PATENTs Pyle et a1. Mar. 1, 194s. 2,565,265 Peterson Aug. 2l, 1951 2,610,303 Ben sept.9,r1952 

